1. Field of the Invention
The present invention relates to a wafer, a wafer polishing apparatus of polishing a wafer to adjust a predetermined thickness, a wafer polishing method, a method of fabricating a piezoelectric vibrator of fabricating a piezoelectric vibrator by utilizing the wafer polishing method, a piezoelectric vibrator fabricated by the fabricating method, an oscillator, an electronic apparatus and a radiowave timepiece having the piezoelectric vibrator.
2. Description of the Related Art
In recent years, a portable telephone or a portable information terminal apparatus uses a piezoelectric vibrator utilizing quartz or the like as a time source, a timing source of a control signal or the like or a reference signal source or the like. As a piezoelectric vibrator of this kind, various ones are known, for example, a piezoelectric vibrator having a piezoelectric vibrating piece of a tuning fork type, a piezoelectric vibrator having a piezoelectric vibrating piece of carrying out a thickness slipping vibration or the like is known.
Meanwhile, the piezoelectric vibrating piece is formed by various piezoelectric members of quartz, lithium tantalite, lithium niobate and the like. Specifically, a raw stone of a piezoelectric member is cut to constitute a wafer, thereafter, the wafer is polished to a predetermined thickness. Further, the polished wafer is cleaned and dried, thereafter, etched by a photolithography technology to form an outer shape of a piezoelectric vibrating piece, and an electrode is formed by patterning a predetermined metal film. Thereby, a plurality of piezoelectric vibrating pieces can be fabricated at a time from a single wafer.
According to the piezoelectric vibrating piece fabricated in this way, a thickness of its own depends on a thickness of a wafer, and therefore, the above-described polishing is made to constitute a particularly important step for determining a quality or the like. Normally, in polishing, a lapping step of roughly polishing a wafer cut from a raw stone to a thickness to some degree, and a polishing step of subjecting the wafer to mirror face polishing to finish highly accurately to a predetermined thickness after the lapping step are carried out. Further, a wafer subjected to such polishing in a shape of a rectangular plate (including square plate shape) near to a state of cutting a raw stone is frequently used as a mode thereof (refer to, for example, JP-A-2006-339896, JP-A-2007-184810).
Further, the polishing is carried out generally by utilizing a general polishing apparatus. Here, a polishing apparatus of a background art will be simply explained in reference to FIG. 28 through FIG. 31.
As shown by FIG. 28 and FIG. 29, a polishing apparatus 200 includes a sun gear 201, an internal gear 202, and a carrier 203. Further, upper and lower sides of the carrier 203 are arranged with an upper surface plate 204 and a lower surface plate 205. Both of the sun gear 201 and the internal gear 202 are rotated in counterclockwise direction, and the carrier 203 is made to carry out a planetary movement. That is, the carrier 203 is revolved in the counterclockwise direction while rotating the carrier 203 in the clockwise direction.
The carrier 203 is constituted by a circular plate of, for example, a diameter of several inches, formed to be sufficiently thin in comparison with a wafer S1 before being polished, and is formed with a holding hole 203 A of holding the wafer S1. As shown by FIG. 30, the holding hole 203A is formed in a square shape or a rectangular shape in an opening thereof in correspondence with the shape of the wafer S1, and is formed in a square shape (rectangular shape) one size larger than the wafer S1 to hold the wafer S1 with play. Further, in the example, only one of the holding hole 203A is formed to the carrier 203 in accordance with large-sized formation of the wafer S1.
Further, a plurality of pieces of the carriers 203 are arranged at respective predetermined angles between the upper surface plate 204 and the lower surface plate 205, and therefore, according to the polishing apparatus 200, a plurality of the wafers S1 can simultaneously be polished by 1 batch of polishing.
The upper surface plate 204 and the lower surface plate 205 are attachably and detachably fixed with polishing pads P at respective faces thereof opposed to each other. Further, the wafer S1 held by the carrier 203 is pinched by the two surface plates 204, 205 from upper and lower sides. At this occasion, the upper surface plate 204 is brought into a state of applying a predetermined load on the wafer S1. The lower surface plate 205 is rotated in the counterclockwise direction constituting a direction reverse to a direction of revolving the carrier 203.
Further, as shown by FIG. 29 and FIG. 31, the upper surface plate 204 is formed with a plurality (several tens) pieces of supply paths 204a penetrating the upper surface plate 204 for supplying a polishing solution W to between the two surface plates 204, 205. Specifically, the supply paths 204a are formed at respective predetermined intervals along an inner side circle of a radius ra, a middle circle of a radius rb, an outer side circle of a radius rc. Further, the polishing solution W is supplied to the supply path 204a by way of a supply hose, not illustrated. Thereby, the polishing solution W can be supplied to between the two surface plates 204, 205 by way of the respective supply paths 204a and the wafer S1 can be polished by utilizing the polishing solution W.
Further, the polishing pad P formed at the upper surface plate 204 is formed with an opening so as not to close the supply path 204a. Further, the polishing solution W mixed with a polishing agent of a small particle size is used. Further, when the lapping step or the polishing step is carried out, normally, different polishing apparatus 200 are used although the polishing apparatus 200 are constructed by the same constitution. However, when lapping is carried out, normally, the polishing pads P are respectively detached from the upper surface plate 204, the lower surface plate 205 and are not used. This is for progressing lapping by finely destructing the wafer surface by a polishing member (isolated abrasive grain).
A simple explanation will be given of a case of polishing the wafer S1 by carrying out a lapping step by the polishing apparatus 200 constituted in this way.
First, the respective carriers 203 are set on the lower surface plate 205 in a state of separating the upper surface plate 204 and the lower surface plate 205. Further, the wafers S1 are respectively set to the holding holes 203A of the respective carriers 203 set in this way. After finishing the setting step in this way, the upper surface plate 204 is moved down and the upper surface plate 204 is pressed to the upper face of the wafer S1 by a predetermined load and the wafer S1 is squeezed by the two surface plates 204, 205.
Further, the carrier 203 is rotated and revolved by driving the sun gear 201, the internal gear 202 while supplying the polishing solution W to between the two surface plates 204, 205 by way of the supply path 204a. Further, the lower surface plate 205 is rotated simultaneously therewith. Thereby, the both faces of the wafer S1 held by the carrier 203 can be polished and the thickness of the wafer S1 can be finished to a predetermined thickness.
However, there is the following problem to be improved in polishing the wafer described above.
With regard to the wafer S1, the large-sized formation is progressed as described above in order to promote mass production performance by increasing a number of pieces taken from a single wafer. On the other hand, in order to meet a request for small-sized formation, also thin thickness formation of the wafer S1 is progressed. In this way, large-sized formation and thin thickness formation are simultaneously requested for the wafer S1, and therefore, when polishing is carried out in the mode shown in FIG. 30, a breakage of a crack or a chip off is liable to be brought about particularly at a corner portion thereof, as a result, yield is reduced.
Here, in polishing, normally, a corner portion of the wafer S1 shown in FIG. 30 is subjected to C chamfering (chamfering) to thereby prevent a crack or a chip off at the corner portion. However, even when the corner portion is chamfered, only a corner portion larger than right angle is formed and the corner portion still remains although the corner portion is small. Therefore, when the corner portion is brought into contact with an inner peripheral face of the holding hole 203A of the carrier 203 in polishing, point (line) contact is brought about, and therefore, by applying a large load thereto, a breakage of a crack or a chip off is liable to be brought about as described above.
That is, large-sized formation of the wafer S1 is progressed as described above, and therefore, when impacted or rubbed to the inner peripheral face of the holding hole 203A of the carrier 203 in polishing, the impact (load) is increased by an amount of large-sized formation. On the other hand, thin thickness formation is progressed, and therefore, the wafer S1 per se becomes brittle by that amount. Therefore, when impacted (loaded) considerably, a breakage of a crack or a chip off is liable to be brought about particularly at the corner portion as described above.
Further, in working a piezoelectric member of quartz or the like constituting a wafer to a piezoelectric vibrating piece, it is necessary to constitute an end face of the wafer by a reference face constituted by having an angle determined to a predetermined face based on a crystal thereof. Therefore, it is important for the wafer to be able to easily identify where such a reference face is.